1. Field of the Invention
The invention relates to a method of making thin poorly soluble coatings on substrates of any desired morphology. In this context, preferably ceramic and oxidic layers, and also metallic as well as further chalcogenidic layers are to be producible as well.
2. The Prior Art.
As defined (see “Technische Keramik” Publisher B. Thier, Vulcan Verlag, Essen 1988, pages 2 to 25) by the Deutsche Keramische Gesellschaft (German Ceramic Society), ceramic materials are inorganic, non-metallic, poorly soluble in water and at least 30% crystalline. They may, however, be extended to the group of glasses, glass ceramics and inorganic binding agents. The ceramic materials are subdivided into the two main groups of “functional ceramics” and “structural ceramics”. Structural ceramics are materials based on oxides and silicates as well as on carbides, nitrides, borides and silicides (MoSi2) of major group elements.
When viewed systematically, “oxide ceramics” would be understood to be all those ceramic materials consisting essentially (>90%) of single-phase and single-component metal oxides. By contrast, all materials based upon ceramically produced materials from the system of boron, carbon, nitrogen, silicon and, in certain circumstances, oxygen are called non-oxide-ceramics. Oxide ceramic materials are polycrystalline materials made from pure oxides or oxide compounds; they are of high purity and are usually free of a vitreous phase. In addition to the high-melting metal oxides, such as, for instance, aluminum-, zirconium-, magnesium-, titanium- and berryllium oxide, and calcium oxide, magneto-ceramic materials and materials of high dielectric constant, piezo-ceramic, may also be included. However, limitation to high-melting oxides is customary. However, silicon dioxide (SiO2) is not classified as oxide ceramic. For this reason and in recognition of further oxides which, while suitable, do not belong to ceramic materials, the invention relates also to the making of ceramic as well as oxidic layers. Furthermore, oxide ceramic materials are distinguished between simple oxides and complex oxides. Among these are, for instance, chromite of coarse structure and perovskites, ferrites and garnets of fine structure.
Hitherto poorly soluble coatings have, for instance, been applied to surfaces by sputtering or vapor deposition, by sol-gel techniques, chemical bath deposition or by metal organic chemical vapor deposition (MOCVD). From the essay “Laser Annealing of Zinc Oxide Thin Film Deposited by Spray-CVD” by G. K. Bhaumik et al., Elsevier Materials Science and Engineering B52 (1998) 25-31, it is known to apply a polycrystalline ZnO film to quartz and silicon substrates by the spray-CVD method. To improve its crystal structure, the applied film may then be heated by laser irradiation. The accumulation of undoped ZnO films by spray pyrolysis with an aqueous solution of zinc nitrate is known from the essay “Optical and Electrical Properties of Undoped ZnO Films Grown by Spray Pyrolysis of Zinc Nitrate Solution” by S. A. Studenikin et al., J. Of Appl. Phys. Vol. 83, No. 4, 15 Feb. 1998, 2104-11). The essence of this essay resides in detecting the relationship between the temperature of the pyrolysis and the structural, electrical and optical properties of the ZnO film. Different temperatures were attained by heating the sample substrate, for instance in nitrogen at 400° C.
During sputtering (for ZnO, see: “Use of a Helicon Wave Excited Plasma of Aluminium-Doped ZnO Thin-Film Sputtering” by K. Yamaya et al., Appl. Phys. Lett. 72(2), 12. January 1998, 235-37) atoms are severed from a metal cathode by impinging ions of a gaseous discharge (“cathode sputtering”). The sputtered metal then precipitates as a uniform layer on a surface. Mono-crystalline ZnO thin-layers may be deposited on c-planar sapphire (see “Plasma Assisted Molecular Beam Epitaxy of ZnO on c-Plane Sapphire: Growth and Characterization” by Y. Chen et al., J. Of Appl. Phys., Vol. 84, No. 7, 1 Oct. 1998, 3912-18) by molecular beam epitaxy using oxygen-containing plasma in the presence of a microwave field. Good quality ZnO films may also be made by direct electro-deposition at a low process temperature from aqueous solutions (see: “Preparation of ZnO Films by Electrodeposition from Aqueous Solution” by S. Poulon et al., 13th Europ. Photovoltaic Solar Energy Conference, 23-27, October 1995. Nice, France, 1750-52). In the sol-gel technique (see: “Microstructure of TiO2 and ZnO Films Fabricated by the Sol-Gel-Method” by Y. Ohya et al., J. Am. Ceram. Soc. 79[4] 825-30 (1996), colloidal solutions present as the sol solidify to a gel by reaction with water and removal of solvents with rigidly adsorbed solvent residue. The gel accumulates on surfaces and may be dried.
In the chemical bath deposition (CBD) (for ZnO/CDs/CIS/Mo structures see: “Effects of Cd-Free Buffer Layer for CuInSe2 Thin Solar Cells” by T. Nii et al., First WCPEC; Dec. 5-9, 1994; Hawaii, 254-57), the two different variants “SILAR method” (Successive Ionic Layer Adsorption and Reaction) and “Chalcogeno-Urea Method” are used in the production of poorly soluble metal chalcogenide layers.
The subject matter of the publication “CuInS2 as an Extremely Thin Absorber in an Eta Solar Cell” by J. Möller et al. (Conference Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 6-10 Jul. 1998, pages 209-211, XP 002110735 Vienna) is a method, based upon the above-captioned methods, for the improved manufacture of thin metal chalcogenide layers naming different material compositions. In this method, a solution of a metal compound is initially applied to a substrate so that ions are deposited thereon. The solvent is then removed from the substrate by a drying process. Thereafter, the deposited ion layer is contacted by a chalcogen hydrogen containing gas to bring about a reaction with the metal ions. Homogenous metal chacoginide layers of constant quality may be produced in a simple manner by this method. Such layers can be applied, for instance, as absorber or buffer layers in solar cells. The closest prior art upon which the present invention is based is this essay.
The prior art method may be called ILGAR (Ionic Layer Gas Reaction) process.